Closed wet grinding circuits



March 31, 1970 ECONDOLlOS 3,503,560

CLOSED WET GRINDING CIRCUITS Filed Sept. 21. 1967 RAW MATERIAL f 1ADJUSTABLE GRINDING 4 FEED MEANS MILL A? f r5 GRANULAR' 8 ,6 WATERMATERJAL CLASSIFIER. v l4 THICKENER l! FLOATATION cmcun- FEED INVENTORELIE CONDO]. I08

8? V V W A TTORNEY United States Patent Int. Cl. B02c 1700, 11/08, 17/02US. Cl. 241-20 4 Claims ABSTRACT OF THE DISCLOSURE The materialdelivered by a grinding mill to a classifier is diluted to bring it tooptimum concentration for classification, and then the classified finematerial to be further treated and the classified coarser material to bereground are separately thickened to the requisite concentrations forsuch operations.

This invention relates to improvements in closed wet grinding circuits.

When the end product from a rod or ball wet grinding circuit is to befiner than a given size, the system is usually run as a closed circuitwith a classifier at the grinding mill delivery to separate out the finematerial of the required size and to return any coarser particles to themill for regrinding. Experience has shown that some of the fine materialalso goes back to the mill, thereby unnecessarily overloading the milland appreciably reducing its output capacity.

It has been found that such recirculation of fine material is due to thefact that the high concentrations of material which are required forsatisfactory grinding do not suit the classifier which can only operateefficiently at moderate concentrations, with the result that theclassifier sorts the material imperfectly. On the other hand, themoderate concentrations which are suitable for the classifier areunsuitable for the grinding process, which needs a very highlyconcentrated product if the grinding process is to take placeefliciently. Further, such moderate classifier concentrations are alsounsuitable for subsequent treatment of the classified fine material, asin a flotation circuit feed.

The primary purpose of this invention is to improve such closed grindingcircuits so as to ensure optimum mill and classifier operatingconditions.

In accordance with the invention this improvement is obtained byseparately and independently controlling the feed concentration to themill, the classifier operating concentration, and the fine materialconcentration at the classifier circuit delivery. This control isexercised so that (l) The material in the classifier is diluted to bringit to optimum classifier operation concentration;

(2) The coarse material discharged from the classifier for regrinding isthickened to give it the concentration required for optimum milloperation; and

(3) The fine ground material discharged from the classifier for furthertreatment is thickened to give it the requisite concentration for suchtreatment.

It has been found that when the aforesaid control is exercised on aclosed grinding circuit there results such efiicient classification thatsubstantially no fine material is fed back into the mill, therebyavoiding unnecessary overloads on the mill. Such control also ensuresthat the concentration of the material fed back into the mill can beselected to enable the latter to operate at its maximum efficiency, andthat the fine ground material is produced at the desired concentrationfor efficient treatment there- 3,503,560 Patented Mar. 31, 1970 of.Studies and tests have shown that if all these conditions are satisfied,a much higher output of fine ground material of the requisite size andconcentration can be achieved.

A better understanding of the invention and its advantages will beobtained from a consideration of the following description when read inconnection with the accompanying drawings which shows schematically aclosed circuit grinding system embodying the invention.

In the drawing, the numeral 1 generally designates a grinding mill intowhich the raw material is fed by adjustable feed means 2 of knownconstruction, and into which the granular material for regrinding is fedthrough a conduit 3 provided with a control valve 15. The groundmaterial is discharged by the grinding mill 1 into a conduit 4 whichconducts such material to a classifier 5 of known construction. Theclassifier 5 sorts and delivers the fine material of the requisite sizeto a conduit 6 and discharges the coarser granular material forregrinding into a conduit 7. Water at a controlled rate is fed to theclassifier 5 from an outside supply 14, through a control valve 16 andconduit 8, to dilute the material in the classifier down to theconcentration required for optimum operating efficiency of such device.As an alternative in certain types of classifiers, the water diluent maybe added to the material as it discharges from the grinding mill outletand before it enters the classifier.

The fine classified material is delivered by the conduit 6 to athickener device 9 of known construction capable of thickening theliquid mixture thereof to the concentration required for subsequentprocessing thereof. The thickened mixture of fine material of thedesired size is discharged from the thickener 9 at a controlled ratethrough a conduit 10 provided with a valve 17 and'to suitable processingapparatus, such as a flotation circuit feed 18. The water dischargedfrom the thickener 9 can be recir culated back into the system by way ofa conduit 12 to the valve 16 and used as part of the dilution water forthe classifier.

The coarser classified material discharged into the conduit 7 isdelivered by the latter to a second thickener device 11 of knownconstruction capable of concentrating the liquid mixture thereof toensure that the mixture of raw material fed into the mill at 2 and thematerial fed into the latter at 3 for regrinding is at the concentrationrequired for optimum mill operation. The water discharging from thethickener 11 can also be recirculated back into the system as dilutionwater for the classifier by way of a conduit 13 connecting such device11 to the valve 16.

By way of example, following is a description of the operation of aninstallation of the type depicted in the drawing. Let it be assumed thatthe grinding mill operates at its peak output capacity when it issupplied with material at a concentration of, say, 65% by weight ofmaterial. The dry material is fed to the mill at a given rate throughthe feed means 2. The coarser classified material to be reground is fedto the mill through the conduit 3 at a rate of about 1 /2 times the rateof feed of the dry raw material. The thickener 11 in the recirculationcircuit therefore, has to be adjusted to discharge material at aconcentration of around 50% in order that the mill will receive aproduct at a concentration of around 65 The ground material delivered bythe mill to the conduit 4 is at the same concentration i.e. 65%, whichis too high for the classifier 5 to operate at maximum efficiency.Because of this, water is fed through the conduit 8 to the classifier atsufiicient rate to reduce the concentration of the material delivered bythe mill 1 to that concentration, say about 20%, by weight, of solids,at which the particular classifier has been found to operate at optimumefficiency to effect excellent sizing and with practically a totalabsence of fine material mixed in with the coarser classified granularmaterial delivered to the conduit 7.

The coarser granular material is discharged by the classifier into theconduit 7 at a concentration of about 25%, by weight, of solids. As suchmaterial passes through the thickener 11 its concentration is increasedto about 50% before it enters the mill 1.

The fine material delivered by the classifier 5 to the conduit 6 is at aconcentration of about by weight, of solids. The concentration of thismaterial is increased to about in the thickener 9 to make it suitablefor subsequent treatment in suitable apparatus 18, such as for example,a flotation circuit.

It has been found that a grinding circuit such as above described canprocess 8 metric tons of ore hourly and recirculate 9.6 metric ton i.e.1.2 times its hourly output. On the other hand, a conventionalinstallation of the same power, with only one mill and one classifier inits circuit, has an hourly processing capacity of only 5 metric tons ofore and recirculates asmuch as 18.5 metric tons, i.e., 3.7 times itshourly output. It will thus be seen that with the use of an installationembodying the invention, there is a gain of over It will be understoodthat the invention is not limited to the above example, and that it isintended to cover any alternative lay outs coming within the scope ofthe appended claims. It will be understood also, that the optimumconcentration rates for the mill and classifier products will becalculated according to the various types of equipment used and thematerials being processed.

What is claimed is:

1. The method of wet grinding solid material which comprises feedingcoarse material to a mill so that its concentration, by weight, in themill is high enough for efficient grinding thereof, then diluting thehighly concentrated ground product from the mill to bring it to optimumclassifier operation concentration and in such diluted conditionseparating the fine ground granular material from the coarser particlesthereof, separately thickening the separated coarser classified materialto increase its conceneration materially, and then feeding the thickenedcoarser classified material and raw material in such proportion into themill as to obtain in the mill the desired high concentration of suchmaterial.

2. The method of wet grinding solid material which comprises feedingcoarse material to a mill so that its concentration, by weight, in themill is high enough for efficient grinding thereof, then diluting thehighly concentrated ground product from the mill to bring it to optimumclassifier operation concentration and in such diluted conditionseparating the fine ground granular material from the coarser particlesthereof, and separately thickening the separated fine materialdischarged from the classifier to increase its concentration materially,

3. In a closed circuit for wet grinding solid material, a grinding mill,means for feeding coarse material to be ground to said mill in aconcentration, by weight, high enough for efficient grinding by saidmill, means for diluting the highly concentrated ground productdischarged from said mill to bring it to optimum classifier operationconcentration, classifier means for separating from the diluted mixtureand separately discharging therefrom the fine ground granular materialand the coarser particles of such material, and means for thickening theseparated coarser classified material to increase its concentrationmaterially, said feeding means including means for feeding the thickenedcoarser classified material and raw material in such proportion into themill as to obtain in the mill the desired high concentration of suchmaterial.

4. In a closed circuit as defined in claim 3, in which said dilutingmeans comprises means for conducting the liquid extracted by saidthickener means to said classifier means.

References Cited UNITED STATES PATENTS 2,125,663 8/1938 Wuensch 24l342,499,347 3/1950 Adams 24134 3,145,935 8/1964 Wilson 24l-24 3,3 52,49911/1967 Campbell 24134 GERALD A. DOST, Primary Examiner US. Cl. X.R.24l61,

